Ferritic stainless steel with improved grain boundary erosion, and manufacturing method thereof

ABSTRACT

Disclosed is a ferritic stainless steel with reduced grain boundary erosion. A ferritic stainless steel with reduced grain boundary erosion according to an embodiment includes, in percent by weight (wt %), 0.005 to 0.1% of C, 0.01 to 1.0% of Si, 0.01 to 1.5% of Mn, 13 to 18% of Cr, 0.005 to 0.1% of N, 0.005 to 0.2% of Al, 0.005 to 0.1% of Ni, 0.003% or less of Mo, 0.05% or less of P, 0.005% or less of S, and the remainder being Fe and impurities, and satisfies an Acl, defined by Formula (1) below, of 900 or more and 990 or less:Ac1=36Cr+90Si+76Mo+760Al+350−(800C+1300N+150Ni+50Mn)  Formula (1):(wherein C, N, Si, Mn, Cr, Ni, Al, Mn, and Mo represent a content (wt %) of each element).

TECHNICAL FIELD

The present disclosure relates to a ferritic stainless steel and amanufacturing method thereof, and more particularly, to a ferriticstainless steel with reduced grain boundary erosion and a manufacturingmethod thereof.

BACKGROUND ART

In general, stainless steels are classified according to chemicalcomponents or metal structures thereof. According to the metalstructures, stainless steels are classified into austenitic (300series), ferritic (400 series), martensitic, and duplex stainlesssteels.

Among them, ferritic stainless steels have higher price competitivenessthan austenitic stainless steels because smaller amounts of expensivealloying elements are contained in the ferritic stainless steels.Ferritic stainless steels have been widely used in kitchen utensils,exterior materials of buildings, home appliances, electronic parts, andthe like due to excellent surface gloss, drawability, and oxidationresistance.

Meanwhile, when ferritic stainless billets are reheated and hot-rolled,a dual phase of ferrite and austenite is formed. The austenite istransformed into a martensite phase in the case where the hot-rolledsteel material is coiled and cooled, and the martensite has a very highhardness and is not easily deformed.

Therefore, an annealing process is performed for ferritic stainlesssteels, as a post process to recrystallize a structure deformed duringhot rolling and to decompose an austenite phase generated during hotrolling into a ferrite phase.

As the annealing process, a continuous annealing method in whichannealing is performed after unwinding ferritic stainless steels isgenerally adopted. However, a batch annealing process in which annealingis performed in an unwound state is performed, instead of the continuousannealing process, for 430 ferritic stainless steels due to easilybreaking properties thereof in the case of unwinding.

During batch annealing, an austenite phase is re-generated at anannealing temperature higher than the austenite transformationtemperature and the re-generated austenite phase is re-transformed intomartensite during cooling, resulting in deterioration of formability andcorrosion resistance.

Therefore, a batch annealing process is performed via heat treatment ata temperature directly below a phase transformation temperature from theaustenite phase into the ferrite phase. In general, because austenitephase transformation occurs at a low temperature of 800 to 850° C., along time (35 to 50 hours) is taken for the batch annealing process forcomplete annealing.

The batch annealing process not only consumes a large amount of energy,but also increases manufacturing costs, thereby deterioratingproductivity. In addition, because a long time is required for the batchannealing process, problems such as delay in delivery due to increasedmanufacturing time may occur.

Meanwhile, when 430 ferritic stainless steels are acid-pickled by acommon acid pickling method after continuous annealing, rather thanbatch annealing, grain boundary erosion occurs due to a grain boundaryCr depletion region caused by precipitation of a Cr carbide duringcooling after hot rolling. In steel materials in which grain boundaryerosion occurs, surface defects are caused during subsequent coldworking and a problem of deterioration of corrosion resistance mayoccur.

RELATED ART DOCUMENT

-   (Patent Document 1) Korean Patent Application Laid-Open Publication    No. 10-2019-0072279A (Published on Jun. 25, 2019)

DISCLOSURE Technical Problem

To solve problems as described above, provided are a stainless steelhaving reduced grain boundary erosion, prepared by a method in which abatch annealing process is omitted, and suitable for continuousannealing and a manufacturing method thereof.

Technical Solution

In accordance with an aspect of the present disclosure to achieve theabove-described objects, a ferritic stainless steel with reduced grainboundary erosion includes, in percent by weight (wt %), 0.005 to 0.1% ofC, 0.01 to 1.0% of Si, 0.01 to 1.5% of Mn, 13 to 18% of Cr, 0.005 to0.10% of N, 0.005 to 0.2% of Al, 0.005 to 0.1% of Ni, 0.003% or less ofMo, 0.05% or less of P, 0.005% or less of S, and the remainder being Feand impurities, and

satisfies an Ac1, defined by Formula (1) below, is 900 or more and 990or less:

Ac1=36Cr+90Si+76Mo+760Al+350−(800C+1300N+150Ni+50Mn)  Formula (1):

(wherein C, N, Si, Mn, Cr, Ni, Al, Mn, and Mo represent a content (wt %)of each element).

In addition, according to an embodiment of the present disclosure, thestainless steel may include, in percent by weight (wt %), 0.4 to 1.0% ofMn and 0.1 to 0.150% of Al.

In accordance with another aspect of the present disclosure, a method ofmanufacturing a ferritic stainless steel with reduced grain boundaryerosion includes: preparing a slab comprising, in percent by weight (wt%), 0.005 to 0.1% of C, 0.01 to 1.0% of Si, 0.01 to 1.5% of Mn, 13 to18% of Cr, 0.005 to 0.1% of N, 0.005 to 0.2% of Al, 0.005 to 0.1% of Ni,0.003% or less of Mo, 0.05% or less of P, 0.005% or less of S, and theremainder being Fe and impurities, and satisfying an Ac1, defined byFormula (1) below, of 900 or more and 990 or less; reheating the slab;rough-rolling the reheated slab and finish-rolling the rough-rolledsteel material; coiling the hot-rolled steel material; continuousannealing the coiled steel material in a temperature range T(A) definedby Formula (2) below; and acid-pickling the continuous annealed steelmaterial,

Ac1=36Cr+90Si+76Mo+760Al+350−(800C+1300N+150Ni+50Mn)  Formula (1):

(wherein C, N, Si, Mn, Cr, Ni, Al, Mn, and Mo represent a content (wt %)of each element)

870° C.≤T(A)≤(Ac1-10)° C.  Formula (2):

In addition, according to an embodiment of the present disclosure, theslab may further include, in percent by weight (wt %), 0.4 to 1.0% of Mnand 0.1 to 0.15% of Al.

In addition, according to an embodiment of the present disclosure, thereheating may be performed in a temperature range of 1,000 to 1,200° C.

In addition, according to an embodiment of the present disclosure, thefinish rolling may be performed in a temperature range of 800 to(Ac1-10)° C.

In addition, according to an embodiment of the present disclosure, thecoiling may be performed in a temperature range of 750 to (Ac1-10)° C.

In addition, according to an embodiment of the present disclosure, thecontinuous annealing may be performed for 3 to 10 minutes.

Advantageous Effects

According to an embodiment of the present disclosure, continuousannealing is possible by adjusting alloying elements, and thus aferritic stainless steel with reduced grain boundary erosion and amanufacturing method thereof may be provided.

DESCRIPTION OF DRAWINGS

FIGS. 1A, 1B, and 1C are optical microscopic images of surfaces toobserve degrees of grain boundary erosion after acid pickling ofhot-rolled and continuous annealed steel sheets.

FIG. 1A is an optical microscopic image showing grain boundary erosionvisible in a form being connected along grain boundaries with a greatwidth.

FIG. 1B is an image showing grain boundary erosion visible in a form oflines without being connected along grain boundaries.

FIG. 1C is an image showing grain boundary erosion visible one somegrain boundary traces.

BEST MODE

A ferritic stainless steel with reduced grain boundary erosion accordingto an embodiment of the present disclosure includes, in percent byweight (wt %), 0.005 to 0.1% of C, 0.01 to 1.0% of Si, 0.01 to 1.5% ofMn, 13 to 18% of Cr, 0.005 to 0.1% of N, 0.005 to 0.2% of Al, 0.005 to0.1% of Ni, 0.003% or less of Mo, 0.05% or less of P, 0.005% or less ofS, and the remainder being Fe and impurities, and

satisfies an Ac1, defined by Formula (1) below, of 900 or more and 990or less:

Ac1=36Cr+90Si+76Mo+760Al+350−(800C+1300N+150Ni+50Mn)  Formula (1):

(wherein C, N, Si, Mn, Cr, Ni, Al, Mn, and Mo represent a content (wt %)of each element).

Modes of the Invention

Hereinafter, embodiments of the present disclosure will be described indetail with reference to the accompanying drawings. The embodiments ofthe present disclosure may, however, be embodied in many different formsand should not be construed as being limited to the embodiments setforth herein. Rather, these embodiments are provided so that thisdisclosure will be thorough and complete, and will fully convey theconcept of the invention to those skilled in the art.

Also, the terms used herein are merely used to describe particularembodiments. An expression used in the singular encompasses theexpression of the plural, unless otherwise indicated. Throughout thespecification, the terms such as “including” or “having” are intended toindicate the existence of features, operations, functions, components,or combinations thereof disclosed in the specification, and are notintended to preclude the possibility that one or more other features,operations, functions, components, or combinations thereof may exist ormay be added.

Meanwhile, unless otherwise defined, all terms used herein have the samemeaning as commonly understood by one of ordinary skill in the art towhich this disclosure belongs. Thus, these terms should not beinterpreted in an idealized or overly formal sense unless expressly sodefined herein. As used herein, the singular forms are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise.

The terms “about”, “substantially”, etc. used throughout thespecification means that when a natural manufacturing and a substanceallowable error are suggested, such an allowable error corresponds thevalue or is similar to the value, and such values are intended for thesake of clear understanding of the present disclosure or to prevent anunconscious infringer from illegally using the disclosure of the presentdisclosure.

A ferritic stainless steel with reduced grain boundary erosion accordingto an embodiment of the present disclosure includes, in percent byweight (wt %), 0.005 to 0.1% of C, 0.01 to 1.0% of Si, 0.01 to 1.5% ofMn, 13 to 18% of Cr, 0.005 to 0.1% of N, 0.005 to 0.2% of Al, 0.005 to0.1% of Ni, 0.003% or less of Mo, 0.05% or less of P, 0.005% or less ofS, and the remainder being Fe and impurities.

Hereinafter, reasons for numerical limitations on the contents ofalloying elements will be described.

A content of carbon (C) is from 0.005 to 0.1%.

C, as an interstitial solid solution strengthening element, increasesstrength of a ferritic stainless steel. When the C content is less than0.005%, an amount of a produced carbide decreases so that sufficientstrength cannot be obtained. However, an excess of C may lower atemperature at which a ferrite phase is transformed into an austenitephase and thus an upper limit of continuous annealing temperature islowered. Therefore, the content of C may be adjusted from 0.005% to0.1%.

A content of silicon (Si) is from 0.01 to 1.0%.

Si, as an alloying element essentially added for deoxidation of a moltensteel during a steel making process, improves strength and corrosionresistance and stabilizes a ferrite phase at the same time. Si may beadded in an amount of 0.01% or more in the present disclosure. However,an excess of Si may deteriorate ductility and formability, and thus anupper limit thereof is controlled to 1.0%.

A content of manganese (Mn) is from 0.01 to 1.5%.

Mn forms uniform scales on a surface layer of a ferritic stainless steelduring heat treatment as an element effective on improving corrosionresistance. However, an excess of Mn may generate Mn-based fumes duringwelding which causes precipitation of an MnS phase, therebydeteriorating elongation. Therefore, a lower limit of the Mn content maypreferably be 0.01%, more preferably 0.4%. An upper limit of the Mncontent may preferably be 1.5%, more preferably 1.0%.

A content of chromium (Cr) is from 13 to 18%.

Cr is an alloying element added to improve corrosion resistance ofstainless steels. However, an excess of Cr causes formation of denseoxide scales during hot rolling resulting in sticking defects andincrease manufacturing costs. Therefore, the Cr content may preferablybe controlled to a range of 13 to 18%.

A content of nitrogen (N) is from 0.005 to 0.1%.

Like carbon, nitrogen (N) is an interstitial solid solutionstrengthening element and improves strength of ferritic stainlesssteels. However, an excess of N may deteriorate impact toughness andformability of steels and lower transformation temperature at which anaustenite phase is transformed to a ferrite phase, thereby lowering anupper limit of the continuous annealing temperature of the presentdisclosure. Therefore, the N content may preferably be controlled to arange of 0.005 to 0.1%.

A content of aluminum (Al) is from 0.005 to 0.2%.

Al, as a ferrite phase-stabilizing element, is a strong deoxidizer thatreduces an oxygen content in molten steels. However, an excess of Al maydeteriorate ductility at room temperature and increases non-metalinclusions causing sliver defects in cold-rolled strips anddeterioration in weldability. Therefore, the Al content may preferablybe controlled to a range of 0.005 to 0.2%. A more preferable lower limitof the Al content may be 0.1%, and an even more preferable upper limitthereof may be 0.15%.

A content of phosphorus (P) is 0.05% or less.

P, as an impurity inevitably contained in steels, is a major causativeelement of grain boundary corrosion during acid pickling ordeterioration of hot workability, and thus, it is preferable to controlthe P content as low as possible. Therefore, the P content maypreferably be controlled to 0.05% or less.

A content of sulfur (S) is 0.005% or less.

S, as an impurity inevitably contained in steels, is a major causativeelement of deterioration of hot workability as being segregated in grainboundaries, and therefore, it is preferable to control the S content aslow as possible. Therefore, the S content may preferably be controlledto 0.005% or less.

A content of nickel (Ni) is from 0.005 to 0.1%.

When added in an amount of 0.005%, Ni has effects on improving corrosionresistance. However, an excess of Ni may increase stability of austeniteand increase manufacturing costs because Ni is an expensive element.Therefore, the Ni content may be controlled to a range of 0.005 to 0.1%.

A content of molybdenum (Mo) is 0.003% or less.

Mo is an element effective on improving corrosion resistance ofstainless steels. However, Mo, as an expensive element, may increasemanufacturing costs and an excess of Mo may cause deterioration ofworkability. Therefore, the Mo content may be controlled to 0.003% orless.

The remaining component of the composition of the present disclosure isiron (Fe). However, the composition may include unintended impuritiesinevitably incorporated from raw materials or surrounding environments.In the present disclosure, addition of other alloy components inaddition to the above-described alloy components is not excluded. Theimpurities are not specifically mentioned in the present disclosure, asthey are known to any person skilled in the art of manufacturing.

In addition, the ferritic stainless steel according to an embodiment ofthe present disclosure may satisfy an Ac1, defined by Formula (1) below,of 900 or more and 990 or less.

Ac1=36Cr+90Si+76Mo+760Al+350−(800C+1300N+150Ni+50Mn)  Formula (1):

(wherein C, N, Si, Mn, Cr, Ni, Al, Mn, and Mo represent a content (wt %)of each element)

The Ac1 refers to an austenite transformation temperature calculated byan alloy composition. In the case of heat treatment at a temperatureequal to or higher than the Ac1, a ferrite phase is transformed into anaustenite phase. Conventionally, a continuous annealing in which heattreatment is performed for a short time by increasing an austenitetransformation temperature by adding an alloy of Ti and Nb has beenperformed.

However, Ti may cause problems such as an increase in manufacturingcosts of stainless steels and occurrence of sliver defects incold-rolled products. In addition, Nb may cause defects in an exteriorappearance due to inclusions and reduction in toughness and may increasein manufacturing costs as in the case of Ti.

According to the present disclosure, an annealing temperature at whichrecrystallization sufficiently occurs during continuous annealing may beobtained by adjusting the Ac1 temperature to 900 or more by controllingcontents of austenite-forming elements such as C and N. Also, accordingto the present disclosure, strength may be increased by forming acarbide and a nitride by adjusting the Ac1 to 990 or less.

The ferritic stainless steel according to an embodiment of the presentdisclosure is manufactured according to a method as described below.

A slab including, in percent by weight (wt %), 0.005 to 0.1% of C, 0.01to 1.0% of Si, 0.01 to 1.5% of Mn, 13 to 18% of Cr, 0.005 to 0.1% of N,0.005 to 0.2% of Al, 0.005 to 0.1% of Ni, 0.003% or less of Mo, 0.05% orless of P, 0.005% or less of S, and the remainder being Fe andimpurities, and satisfying an Ac1, defined by Formula (1) below, of 900or more and 990 or less is prepared. The slab is reheated. The reheatedslab is rough-rolled and finish-rolled. The hot-rolled stainless steelis coiled. The coiled hot-rolled stainless steel is continuouslyannealed in a temperature range T(A) defined by Formula (2) below. Thecontinuous annealed steel material is acid-pickled.

Ac1=36Cr+90Si+76Mo+760Al+350−(800C+1300N+150Ni+50Mn)  Formula (1):

(wherein C, N, Si, Mn, Cr, Ni, Al, Mn, and Mo represent a content (wt %)of each element)

870° C.≤T(A)≤(Ac1-10)° C.  Formula (2):

Reasons for numerical limitations on the contents of alloying elementsare as described above.

As an annealing process of a stainless steel-manufacturing process,continuous annealing and batch annealing are used. In general,austenitic stainless steels are annealed by continuous annealing andferritic and martensitic stainless steels are annealed by batchannealing, according to properties of stainless-steel materials of steeltypes.

Upon comparison of annealing processes of hot-rolled coils betweenstainless steel types, continuous annealing is performed by heattreatment at a high temperature (about 900 to 1150° C.) underatmospheric conditions for a short time (about 3 minutes). Unlike thisprocess, batch annealing is performed by heat treatment at a lowtemperature (about 750 to 850° C.) in an ambient gas (hydrogen or mixedgas of nitrogen and hydrogen) for a long time (about 50 hours). Inaddition, because the batch annealing is performed in a coiled state, adifference in properties of a material may occur between areas due todeviation of annealing temperature at different areas of a hot-rolledcoil.

Meanwhile, ferritic stainless steels are annealed to re-incorporate anaustenite phase (martensite phase during cooling) formed after rollinginto a ferrite phase and to remove stress generated during hot rollingto facilitate cold rolling.

When an annealing temperature increases above the austenitetransformation temperature while a ferritic stainless steel is annealed,the austenite phase is re-generated during the annealing. There-generated austenite phase is re-transformed into martensite duringcooling, resulting in deterioration of formability and corrosionresistance of steel materials. Therefore, ferrite stainless steels areproduced by applying a batch annealing process that proceeds at a lowtemperature.

As described above, batch annealing provides lower productivity andlower energy efficiency than continuous annealing. Also, a steelmaterial produced by batch annealing has inferior quality than a steelmaterial produced by continuous annealing due to different annealingtemperatures in different areas. Therefore, there is a need to improve abatch annealing process in a process of manufacturing ferritic stainlesssteels.

According to an embodiment of the present disclosure, a method ofmanufacturing a ferritic stainless steel in which continuous annealingis possible for a hot-rolled steel sheet may be provided. In thisregard, the continuous annealing is performed in the temperature rangeT(A) of 870 to (Ac1-10)° C.

In the case where an annealing temperature is lower than 870° C.,recrystallization does not sufficiently occur. At an annealingtemperature of Ac1 or higher, an austenite phase is formed. Therefore,there is a need to control the continuous annealing temperature to arange of 870 to (Ac1-10)° C.

In addition, according to an embodiment of the present disclosure, thereheating may be performed in a temperature range of 1000 to 1200° C.

When a reheating temperature is low, a rolling load of hot rollingincreases and flaws may be generated in billets during hot rolling. Inaddition, at a low reheating temperature, coarse precipitates generatedduring casting the slab cannot be re-decomposed. Therefore, in thepresent disclosure, a lower limit of the reheating temperature isadjusted to 1000° C.

When the reheating temperature is high, billets may be softened so thata shape changes and coarsening of internal crystal grains cannot beprevented. Therefore, in the present disclosure, an upper limit of thereheating temperature is adjusted to 1200° C.

In addition, according to an embodiment of the present disclosure, thefinish rolling may be performed at a temperature of 800 to (Ac1-10)° C.

When a finish rolling temperature is below 800° C., flaws may begenerated in steel materials and non-uniform structure is formedresulting in deterioration of toughness and strength. At a finishrolling temperature above (Ac1-10)° C., austenite crystal grains arecoarsened, and ferrite grain refinement is not sufficiently performedafter transformation.

In addition, according to an embodiment of the present disclosure, thecoiling is performed at a temperature of 750 to (Ac1-10)° C.,

A preferable coiling temperature is 750° C. or higher for plate shapeand surface quality. A coiling temperature above (Ac1-10)° C. maycorrespond to an austenite phase region, a martensite phase may begenerated during cooling.

In addition, according to an embodiment of the present disclosure, thecontinuous annealing may be performed for 3 to 10 minutes.

When an annealing time is too short, recrystallization is notsufficiently performed. When the annealing time is too long, grain sizeincreases resulting in deterioration of mechanical properties. Inconsideration thereof, the annealing time may be controlled to a rangeof 3 to 10 minutes.

Meanwhile, surface gloss is an important property of stainless steels,and various manufacturing methods are used to improve the surface gloss.Particularly, because annealing acid pickling is not performed aftercold rolling in the case of high-gloss products, a surface before coldrolling remains even after the cold rolling. Therefore, it is importantto control the shape of the surface before cold rolling.

For example, grain boundary erosion, scale residues, or the like, whichare caused by excessive acid pickling to remove scales of hot-rolled,annealed stainless steel sheets, form a very rough surface after the hotrolling and acid pickling, and thus surface quality is still inferioreven after cold rolling.

In a shape of grain boundary erosion occurring on the surface of astainless steel, grain boundaries are folded during cold rolling and maycause surface defects called gold dust defects in final products.

According to an embodiment of the present disclosure, the ferriticstainless steel manufactured under control conditions of the presentdisclosure does not have surface grain boundary erosion after acidpickling because continuous annealing temperature is controlled inrelation to the austenite phase transformation temperature (Ac1).

Hereinafter, the present disclosure will be described in more detailthrough examples. However, it is necessary to note that the followingexamples are only intended to illustrate the present disclosure in moredetail and are not intended to limit the scope of the presentdisclosure. This is because the scope of the present disclosure isdetermined by matters described in the claims and able to be reasonablyinferred therefrom.

EXAMPLES

Slabs having compositions of alloying elements shown in Table 1 belowwere prepared by continuous casting and reheated in a temperature rangeof 1,000 to 1,200° C. The reheated slabs were rough-rolled andfinish-rolled using a finish rolling mill at a finish rollingtemperature of 800° C., and then coiled at 750° C.

TABLE 1 Alloying elements (wt %) Category C Si Mn Cr Ni Al N Ac1 RemarksSteel A 0.06 0.20 0.80 16.29 0.1 0.08 0.023 880 Comparative Steel SteelB 0.04 0.20 0.50 16.25 0.1 0.10 0.013 940 Inventive Steel Steel C 0.030.32 0.40 16.20 0.1 0.12 0.010 981 Inventive Steel

The coiled steel materials were continuous annealed for 10 minutes underthe annealing temperature conditions shown in Table 2 below.Subsequently, the hot-rolled and continuously annealed steel sheets weredescaled with a short blaster, primarily acid-pickled in a sulfuric acidsolution, and then acid-pickled in a mixed acid solution (nitricacid+hydrofluoric acid).

Table 2 below shows degrees of grain boundary erosion with respect tochanges in the continuous annealing temperature after acid pickling.

The grain boundary erosion was graded into a case in which grainboundary erosion occurred with a great width in a form of beingconnected along grain boundaries (severe grain boundary erosion) asshown in FIG. TA, a case in which grain boundary erosion occurred in aform of lines without being connected along the grain boundaries(moderate grain boundary erosion) as shown in FIG. 1B, and a case inwhich lines were visible on some grain boundary traces (weak grainboundary erosion).

The severe grain boundary erosion was marked by ‘O’, the moderate grainboundary erosion was marked by ‘-’, and weak grain boundary erosion wasmarked by ‘X’.

TABLE 2 Continuous annealing Degree of grain Annealing boundary CategorySteel type temperature (° C.) Ac1-10 erosion Example 1 Steel B 870 930 XExample 2 Steel B 900 930 X Example 3 Steel B 930 930 X Example 4 SteelC 870 971 X Example 5 Steel C 900 971 X Example 6 Steel C 930 971 XExample 7 Steel C 960 971 X Comparative Steel A 810 870 — Example 1Comparative Steel A 840 870 — Example 2 Comparative Steel A 870 870 XExample 3 Comparative Steel A 900 870 ◯ Example 4 Comparative Steel A930 870 ◯ Example 5 Comparative Steel A 960 870 ◯ Example 6 ComparativeSteel A 990 870 ◯ Example 7 Comparative Steel B 810 930 — Example 8Comparative Steel B 840 930 — Example 9 Comparative Steel B 960 930 ◯Example 10 Comparative Steel B 990 930 ◯ Example 11 Comparative Steel C810 971 — Example 12 Comparative Steel C 840 971 — Example 13Comparative Steel C 990 971 — Example 14

Referring to Table 2, the Ac1-10 value of Inventive Steel B was 930 andthe Ac1-10 value of Inventive Steel C was 971. In Examples 1 to 3,Inventive Steel B was continuous annealed in a temperature range of 870to 930° C. In Examples 4 to 7, Inventive Steel C was continuous annealedin a temperature range of 870 to 971° C. Because Examples 1 to 7satisfied the alloy compositions, Ac1 values, and continuous annealingtemperatures suggested in the present disclosure, weak grain boundaryerosion occurred.

On the contrary, in Comparative Examples 1 and 2, the continuousannealing was performed at 810° C. and 840° C., respectively, which arebelow 870° C., and thus moderate grain boundary erosion occurred.

Although weak grain boundary erosion occurred in Comparative Example 3,the austenite transformation temperature was low since the Ac1 value was880 which was below 900. Therefore, a temperature range of ComparativeExample 3 is limited during the process and recrystallization isdifficult to be sufficiently performed.

Because the continuous annealing was performed at a temperature abovethe Ac1-10 in Comparative Examples 4 to 7, severe grain boundary erosionoccurred.

Because Inventive Steel B was continuous annealed at a temperature below870° C. in Comparative Examples 8 and 9, moderate grain boundary erosionoccurred.

In Comparative Examples 10 and 11, Inventive Steel B was continuousannealed at 960° C. and 990° C., respectively. Because the continuousannealing temperature of Comparative Examples 10 and 11 exceeded theAc1-10 value of 930, severe grain boundary erosion occurred.

Because Inventive Steel B was continuous annealed at temperature below870° C. in Comparative Examples 12 and 13, moderate grain boundaryerosion occurred.

In Comparative Example 14, Inventive Steel C was continuous annealed at990° C. Because the continuous annealing temperature of ComparativeExample 14 exceeded the Ac1-10 value of 971, moderate grain boundaryerosion occurred.

According to the embodiments, when the continuous annealing temperaturewas below 870° C., moderate grain boundary erosion occurred, and weakergrain boundary erosion occurred at a higher continuous annealingtemperature. However, when the continuous annealing temperature exceededthe (Ac1-10)° C., severe grain boundary erosion occurred.

While the present disclosure has been particularly described withreference to exemplary embodiments, it should be understood by those ofskilled in the art that various changes in form and details may be madewithout departing from the spirit and scope of the present disclosure.

INDUSTRIAL APPLICABILITY

The ferritic stainless steel according to the present disclosure hasreduced grain boundary erosion by applying continuous annealing, andaccordingly batch annealing may be omitted. Therefore, manufacturingcosts are reduced, and the industrial applicability of the presentdisclosure is considered high.

1. A ferritic stainless steel with reduced grain boundary erosioncomprising, in percent by weight (wt %), 0.005 to 0.1% of C, 0.01 to1.0% of Si, 0.01 to 1.5% of Mn, 13 to 18% of Cr, 0.005 to 0.1% of N,0.005 to 0.2% of Al, 0.005 to 0.1% of Ni, 0.003% or less of Mo, 0.05% orless of P, 0.005% or less of S, and the remainder being Fe andimpurities, wherein an Ac1, defined by Formula (1) below, is 900 or moreand 990 or less:Ac1=36Cr+90Si+76Mo+760Al+350−(800C+1300N+150Ni+50Mn)  Formula (1):(wherein C, N, Si, Mn, Cr, Ni, Al, Mn, and Mo represent a content (wt %)of each element).
 2. The ferritic stainless steel according to claim 1,wherein the ferritic stainless steel comprises, in percent by weight (wt%), 0.4 to 1.0% of Mn and 0.1 to 0.15% of Al.
 3. A method ofmanufacturing a ferritic stainless steel with reduced grain boundaryerosion, the method comprising: preparing a slab comprising, in percentby weight (wt %), 0.005 to 0.1% of C, 0.01 to 1.0% of Si, 0.01 to 1.5%of Mn, 13 to 18% of Cr, 0.005 to 0.1% of N, 0.005 to 0.2% of Al, 0.005to 0.1% of Ni, 0.003% or less of Mo, 0.05% or less of P, 0.005% or lessof S, and the remainder being Fe and impurities, and satisfying an Ac1,defined by Formula (1) below, of 900 or more and 990 or less; reheatingthe slab; rough-rolling the reheated slab and finish-rolling therough-rolled steel material; coiling the hot-rolled steel material;continuous annealing the coiled steel material in a temperature rangeT(A) defined by Formula (2) below; and acid-pickling the continuousannealed steel material,Ac1=36Cr+90Si+76Mo+760Al+350−(800C+1300N+150Ni+50Mn)  Formula (1):(wherein C, N, Si, Mn, Cr, Ni, Al, Mn, and Mo represent a content (wt %)of each element)870° C.≤T(A)≤(Ac1-10)° C.  Formula (2):
 4. The method according to claim3, wherein the slab comprises, in percent by weight (wt %), 0.4 to 1.0%of Mn and 0.1 to 0.15% of Al.
 5. The method according to claim 3,wherein the reheating is performed in a temperature range of 1,000 to1,200° C.
 6. The method according to claim 3, wherein the finish rollingis performed in a temperature range of 800 to (Ac1-10)° C.
 7. The methodaccording to claim 3, wherein the coiling is performed in a temperaturerange of 750 to (Ac1-10)° C.
 8. The method according to claim 3, whereinthe continuous annealing is performed for 3 to 10 minutes.